The present invention relates to an apparatus for manufacturing a compound semiconductor single crystal by pulling it according to the LEC (Liquid Encapsulated Czochralski) method.
The LEC method has recently been proposed as a method of manufacturing a single crystal of a compound semiconductor having a high decomposition pressure at a melting point such as GaAS, GaP or InP. This method involves crystal growth at high temperature and high pressure, i.e., a strict thermal environment for crystal growth. For this reason, stable crystal growth and control of crystal quality may be difficult to attain.
A conventional apparatus for manufacturing a compound semiconductor single crystal utilizing the LEC method will briefly be described with reference to FIG. 11. A crucible 120 supported by a crucible holder 130 is disposed in a high-pressure container 110 and is heated by a heat generator 140 coaxially surrounding it. Upon heating by the heat generator 140, a raw material melt 161 and a liquid encapsulating material 162 covering it, are formed in the crucible 120.
Decomposition of the raw material melt 161 at high temperatures is controlled by the liquid encapsulating material 162 and a pressurized inert gas 167. A single crystal 165 is formed through the liquid encapsulating material 162 by bringing a seed crystal into contact with the melt 161 in the crucible 120 and pulling the seed crystal while rotating a pulling rod.
A heat insulator assembly 150 (heat insulators 151, 152, and 153) is arranged around the circumferential and upper and lower surfaces of the heat generator 140 so as to guarantee good heating efficiency of the heat generator 140, and to maintain optimal temperature distribution in the crucible 120. The heat insulators normally comprise carbon members.
However, such carbon products are easily oxidized and degraded by oxygen or water vapor in an atmosphere gas. Consequently, a satisfactory heating efficiency and optimal temperature distribution in the crucible cannot be obtained when the crucible 120 is heated by the heat generator 140. In addition, the raw material melt is contaminated by oxides formed by oxidation. These factors degrade the manufacturing yield and quality of single crystals manufactured by the LEC method.
For example, GaAs single crystals, which are recently receiving public attention as ultra high-speed IC substrates, contain carbon as a p-type impurity in the amount of 1.times.10.sup.16 cm.sup.-3 or more for the reasons described above. This has presented a big problem in manufacturing an un-doped, uniform, semiinsulating substrate with good reproducibility and without thermal degradation.
In view of this problem, it has been proposed to coat the carbon heat insulator assembly 150 with a thin film of PBN, Si.sub.3 N.sub.4 or SiC by the CVD method, so as to prevent the above-mentioned contamination. However, in a heat insulator having such a thin film, film degradation is significant after repeated use at high temperature and pressure since the carbon and film have different coefficients of thermal expansion and the film has only a limited density. Moreover, the composition of the thin film can easily contaminate the raw material melt. For these reasons, it has been difficult to manufacture uniform, semiinsulating substrates with good reproducibility.
The present inventors experimentally manufactured a heat insulator assembly 150 of AlN, Al.sub.2 O.sub.3, ZrO.sub.2 or the like, in place of carbon in an attempt to prevent contamination by carbon. The results revealed that the problems encountered with the CVD thin film could be prevented, and contamination with carbon could be eliminated. However, when one of the carbon replacing substances proposed above is sintered, in order to maintain a satisfactory mechanical strength of final products an alkali earth metal oxide (e.g., CaO or MgO) or a rare earth element oxide (e.g., Y.sub.2 O.sub.3 or La.sub.2 O.sub.3) must be added in an amount of a few % as a binder. This provides another contamination source to the raw material melt by smearing out of the binder component upon repeated use at high temperature and high pressure. In particular, Ca or Mg is an impurity which can form an electrically active level, therefore, the use of Ca or Mg presents a problem in manufacturing a uniform, semiinsulating substrate with good reproducibility. Furthermore, the above-mentioned substances have thermal conductivities lower than that of carbon, which is generally used. For example, AlN has a thermal conductivity of about 50 W/m.K and Al.sub.2 O.sub.3 has a thermal conductivity of about 20 W/m.K, which are less than 1/10 that of carbon. For this reason, a heat insulator assembly 150, formed of one of the above-mentioned substances, has a non-uniform temperature distribution and is easily damaged by thermal stress. The above situation can be summarized by noting that a large amount of a binder must be used in order to increase mechanical strength, yet that contamination of a raw material melt increases when a large amount of a binder is used.
In the heat insulator assembly 150, the upper heat insulator 152 shown in FIG. 12 has a particularly low thermal symmetry as compared with the side heat insulator 151 and the lower heat insulator 153. In addition, since the surface of the upper heat insulator 152 is directly cooled by convection of the pressurized inert gas 167, a large temperature difference exists between the inner and outer diameter portions. Therefore, the upper heat insulator 152, in particular, in the heat insulator assembly 150, suffers from the problem of easy damage by thermal stress.
In view of the problems discussed above, it has been difficult to use a sintered body of AlN, Al.sub.2 O.sub.3, ZrO.sub.2 or the like, for a heat insulator assembly, in particular, an upper heat insulator having satisfactory thermal resistance and mechanical strength while preventing carbon contamination of the raw material melt to thereby maintain the high purity of the melt.